This invention relates generally to the detection of chemical signals that are diffused through a solid or semi-solid, or quiescent liquid surface, particularly where the chemical signals are associated with a medically important molecule.
An electrode is the component in the electrochemical cell in contact with an electrolyte through which current can flow by electronic movement. Electrodes, which are essential components of both galvanic (current producing) and electrolytic (current using) cells, can be composed of a number of electrically conductive materials, e.g., lead, zinc, aluminum, copper, iron, nickel, mercury, graphite, gold, or platinum. Examples of electrodes are found in electric cells, where they are dipped in the electrolyte; in medical devices, where the electrode is used to detect electrical impulses emitted by the heart or the brain; and in semiconductor devices, where they perform one or more of the functions of emitting, collecting, or controlling the movements of electrons and ions.
The electrolyte can be any substance that provides ionic conductivity, and through which electrochemically active species can be transported (e.g., by diffusion). Electrolytes can be solid, liquid, or semi-solid (e.g., in the form of a gel). Common electrolytes include sulfuric acid and sodium chloride, which ionize in solution. Electrolytes used in the medical field must have a pH which is sufficiently close to that of the tissue in contact with the electrode (e.g., skin) so as not to cause harm to the tissue over time.
Electrochemically active species that are present in the electrolyte can undergo electrochemical reactions (oxidation or reduction) at the surface of the electrode. The rate at which the electrochemical reactions take place is related to the reactivity of the species, the electrode material, the electrical potential applied to the electrode, and the efficiency at which the electrochemically active species is transported to the electrode surface.
In unstirred electrolyte, such as quiescent liquid solutions and gel electrolytes, diffusion is the main process of transport of electrochemically active species to the electrode surface. The exact nature of the diffusion process is determined by the geometry of the electrode (e.g., planar disk, cylindrical, or spherical), and the geometry of the electrolyte (e.g., semi-infinite large volume, thin disk of gel, etc.). For example, diffusion of electrochemically active species to a spherical electrode in a semi-infinite volume of electrolyte differs from diffusion of electrochemically active species to a planar disk electrode. A constant and predictable pattern of diffusion (i.e., a diffusion pattern that can be predicted by a simple equation) is critical in determining a correlation between the electrochemical current collected, and the concentration of the electrochemically active species in the electrolyte.
However, diffusion of electrochemically active species toward an electrode can not be predicted by a simple equation for every situation. For example, where the electrochemically active species diffuses through a disk-shaped electrolyte toward a smaller disk-shaped electrode in contact with the electrolyte, the current observed at the electrode can not be predicted by a simple equation. In this latter situation, the inaccuracy in the diffusion model is caused by the combination of two different diffusion models. First, in the center of the disk electrode the diffusion of the electroactive species towards the electrode is in a substantially perpendicular direction. Secondly, at the edges of the disk electrode the diffusion comes from both perpendicular and radial directions. The combination of these two different diffusion patterns makes the total current collected at the disk electrode difficult to predict. In addition, the relative contributions of the diffusion fluxes from the axial and radial directions may change over time, causing further errors in predicted current.
A mask which is substantially impermeable to the transport of a chemical signal is positioned in the chemical signal transport path moving toward a working electrode which senses an electrochemical signal diffused through a material which is ionically conductive, which material comprises water and an electrolyte. More particularly, the mask of the invention is positioned on or in the ionically conductive material, such as an ion-containing gel, between an area from which the chemical signal is transported and the catalytic face of the working electrode used to sense the chemical signal. The configuration of the mask (e.g., shape, thickness, mask component(s)) is such that the mask prevents substantially all chemical signal transport (from the chemical signal source) having a radial vector component relative to a plane of the mask and the catalytic face of the working electrode, thus allowing primarily only chemical signal transport (e.g., diffusion) that is substantially perpendicular to the place of the mask and the catalytic surface of the working electrode. The mask thus minimizes radial transport of the chemical signal to the working electrode and accumulation of chemical signal at the periphery of the working electrode. The mask thus significantly reduces or eliminates edge effects, since the chemical signal that reaches the electrode is primarily only that chemical signal that is transported in a direction substantially perpendicular to the catalytic face of the working electrode. Substantially all transport of chemical signal to the working electrode surface via a path which includes an radial vector component (i.e., is not a path substantially perpendicular to the working electrode catalytic surface) is prevented from occurring by the mask, since the mask blocks entry of potentially radially transported chemical signal at the source. By substantially reducing edge effects created by radial transport of chemical signal, it is possible to obtain a more accurate measurement of the amount (e.g., concentration) of chemical signal that is transported from a given area of source material.
In one embodiment, the working electrode is a closed polygon or closed circle. The mask has an outer perimeter which is equal to or greater than (i.e., extends beyond) the outer perimeter of the working electrode. The mask has an opening, the opening being sufficiently small so that chemical signal that passes through the opening to the catalytic surface of the working electrode in a direction that is substantially perpendicular to the plane of the mask, and thus, substantially perpendicular to the working electrode catalytic face.
In another embodiment, the working electrode is annular and the mask is composed of a solid, circular piece concentrically positioned with respect to the working electrode such that the outer perimeter of the solid circular piece is circumscribed substantially within the inner perimeter of the annular working electrode. Thus chemical signal that passes with the electrolyte flow and through the plane of the mask is substantially only that chemical signal that is transported from the chemical signal source in a direction that is substantially perpendicular to the working electrode catalytic face.
In another embodiment, the mask is attached to a surface of a hydrogel patch, and the mask and hydrogel patch are provided as a single unit.
In another embodiment, the mask is an integral part of the housing for the sensor portion of a device for monitoring the chemical signal.
In another embodiment, the mask is independent of any portion of the device with which it is to be used, i.e., the mask is not bound to another component but merely placed, by the user, in contact with the electrolyte containing material prior to use.
An object of the invention is to provide a means that can be used with virtually any surface-contacting working electrode, and can enhance the performance of the electrode and the accuracy of measurements from the working electrode.
Another object of the invention is to provide a means for accurately and consistently measuring the amount of a chemical signal present in a sample by minimizing the error created by chemical signal which moves to the electrode with a radial vector component.
Another object of the invention is to provide a means for quickly, accurately, and continually measuring a chemical signal transported through an electrolyte containing ion conducting material, e.g., a hydrogel. By using the mask of the invention with a working electrode as described herein, measurement of the chemical signal transported through the material in a path perpendicular to the electrode is achieved within a matter of seconds to minutes.
An object of the invention is to provide a disposable assembly which makes it possible to proportionally measure a chemical signal by conversion into an electrical signal, where the electrical signal can be measured and accurately correlated with the amount of chemical signal present in a given source (e.g., the amount of chemical signal present in and/or below a section of skin, or in a hydrogel patch in contact with the working electrode).
An advantage of the invention is that use of the mask with a working electrode and hydrogel allows for measurement of very small amounts of an electrochemical signal. For example, the mask of the invention can be used in conjunction with a working electrode, electroosmotic electrode and hydrogel system for monitoring glucose levels in a subject. An electroosmotic electrode (e.g., reverse iontophoresis electrode) can be used to electrically draw glucose into the hydrogel. Glucose oxidase (GOD) contained in the hydrogel catalyzes the conversion of glucose to generate a reaction product (hydrogen peroxide) which can be proportionally converted to an electrical signal. The electroosmotic electrode is switched off and the working electrode of the invention is turned on. The working electrode catalyzes the resulting chemical signal into an electrical signal which is correlated to the amount of chemical signal. This system allows for the continuous and accurate measurement of an inflow of a very small amount of glucose (e.g., glucose concentrations 10, 500, or 1,000 or more times less than the concentration of glucose in blood).
Another advantage is that the mask is easily and economically produced and is disposable.
A feature of the mask and the device used therewith is that it is flat (e.g., a disk with substantially planar surfaces), thin (e.g., 0.5 to 10 mils), impermeable to chemical signal flow with the electrode/mask/hydrogel assembly having a surface area on each face in the range of about 0.5 cm2 to 10 cm2.
A feature of the mask is that where the mask includes an opening, the opening has substantially the same dimensions or smaller as the working electrode i.e., the outer perimeter of the opening is circumscribed within the outer perimeter of the catalytic surface of the working electrode.
These and other objects, advantages and features of the present invention will become apparent to those persons skilled in the art upon reading the details of the composition, components and size of the invention as set forth below reference being made to the accompanying drawings forming a part hereof wherein like numbers refer to like components throughout.